A Tertiary dolerite plug at Tieveragh, Co. Antrim cuts Old Red Sandstone tuffaceous sedimentary rocks. Glassy buchites have formed by disequilibrium partial melting induced by convective heat transfer from the basaltic magma. Variation in bulk rock chemistry has produced glasses coexisting with variable proportions of pyroxene, cordierite, ilmenite, hematite, magnetite, mullite, plagioclase, and alkali feldspar. Mixing of the basaltic magma with acid melt illustrates how up to 10% contamination may occur without radically altering the course of crystallization. Mineral geothermometers indicate maximum temperatures at the contact around 1100°C falling to only 940° at 10 m.